The United Kingdom cannot ignore the defence challenges facing the country and "retreat into isolation", the House of Commons Defence Committee has warned.
The committee has published a report which attacks the planning, structures and assumptions that drove defence policy and spending decisions for the past five years.
The report says: "For the first time in 20 years, an advanced military state has challenged the borders of European nations, and the security challenges in the Middle East, Africa, and South Asia have increased dramatically in scale and complexity.
"Russia has annexed Crimea, and Russian-backed separatists have taken much of Eastern Ukraine.
"DAESH (or ISIL) have seized the second largest city in Iraq, and now control areas of a territory larger than the United Kingdom.
It adds: "Over 10,000 civilians were casualties in Afghanistan last year. Serious instability persists in Darfur, Somalia, the Central African Republic, and Pakistan.
"Three million people have been displaced and two hundred thousand killed in Syria."
As a result of this instability, defence policy must be adjusted to cope, the committee said.
With an eye on Russia, the committee said: "The UK must rebuild its conventional capacities eroded since the Cold War.
"The requirements are many, including Maritime Surveillance, Nuclear, Biological, Chemical and Radiological warfare training, developing a Ballistic Missile Defence capability, an enhanced Navy and Air Force, a comprehensive carrier strike capability, and full manoeuvre warfare capacity.
The committee, which is chaired by Rory Stewart, a Tory, said Russia "can deploy 150,000 troops at 72 hours' notice. NATO on current planning would take six months".
It added that the United States pays for 70% of NATO's budget and the UK was soon to fall below the required membership contribution threshold of 2% of GDP.
Britain's defence budget has steadily declined from around 4% of GDP 25 years ago.
Plans to go ahead with the construction of a second aircraft carrier should be shelved amid defence cuts while greater effort should be put into maritime surveillance, the report said.
Five years ago, during the writing of the last Strategic Defence and Security Review, the Government assumed that the days of fighting a sophisticated war against a nation like Russia were over.
Britain's armed forces were cut and are being trained for one quick reaction deployment and one longer-term operation.
Technically the British Army fails the definition of an army, which would normally be three-corps strong, as opposed to the one that can be mustered today.
There are fewer people in the British armed forces than the US Marine Corps.
But Defence Secretary Michael Fallon described suggestions that Britain needs to rebuild its defence capabilities as "nonsense".
"Under this government we have gone from the £38bn black hole in the defence budget that we inherited to a properly funded £34bn annual budget," he said.
"That means we have been able to commit to spending over £160bn on equipment over the next decade to keep Britain safe."
